Prenatal diagnosis (PND) of Primary Immunodeficiency Diseases (PIDs) is indicated in families with a previously affected child where the specific molecular or biochemical defect has been identified. With the shift toward molecular genetics, functional assays are now reserved for cases where the genotype is unknown.
Indications
- Family history of a known PID with high morbidity/mortality.
- Identification of a specific gene mutation in the index case or carrier status in parents.
- Lack of curative options or need for early hematopoietic stem cell transplantation (HSCT) planning.
Modalities of Prenatal Diagnosis
PND can be categorized into Molecular, Biochemical, and Functional/Phenotypic methods.
1. Molecular Diagnosis (Gold Standard)
- Chorionic Villus Sampling (CVS):
- Timing: 10–12 weeks of gestation.
- Procedure: Ultrasound-guided aspiration of trophoblastic tissue.
- Utility: DNA extraction for Targeted Mutation Analysis or Linkage Analysis (RFLP).
- Common Targets: IL2RG (X-SCID), WAS (Wiskott-Aldrich), BTK (X-linked Agammaglobulinemia), CYBB (CGD).
- Amniocentesis:
- Timing: 15–18 weeks of gestation.
- Utility: Culture of amniocytes for DNA analysis. Lower risk of maternal cell contamination compared to CVS but performed later in pregnancy.
2. Biochemical Assays
Used for metabolic PIDs where the enzyme deficiency is expressed in fetal tissues.
- Adenosine Deaminase (ADA) Deficiency: Measured in cultured amniocytes or chorionic villi.
- Purine Nucleoside Phosphorylase (PNP) Deficiency: Enzyme activity assays in fetal cells.
3. Fetal Blood Sampling (Cordocentesis)
- Timing: 18–22 weeks of gestation.
- Method: Ultrasound-guided percutaneous umbilical blood sampling (PUBS).
- Applications:
- Flow Cytometry: Quantification of T, B, and NK cell subsets (e.g., absence of T cells in SCID, absence of B cells in XLA).
- Functional Assays: Nitroblue Tetrazolium (NBT) test or Dihydrorhodamine (DHR) 123 flow cytometry for Chronic Granulomatous Disease (CGD).
- Expression Studies: Assessment of HLA class II expression (Bare Lymphocyte Syndrome) or CD11/CD18 expression (LAD Type 1).
Specific PID Considerations
| Disease | Diagnostic Method | Key Marker/Defect |
|---|---|---|
| SCID | CVS/Amniocentesis | Mutation in IL2RG, RAG1/2, JAK3 |
| XLA | CVS/Fetal Blood | BTK mutation / Absence of B-cells (CD19+) |
| CGD | Fetal Blood (DHR) | Impaired oxidative burst |
| Wiskott-Aldrich | CVS/Fetal Blood | WAS mutation / Small, low platelets |
| LAD-1 | Fetal Blood | Absence of CD18/CD11 |
| Bare Lymphocyte | CVS/Fetal Blood | Absence of HLA Class I/II |
Preimplantation Genetic Diagnosis (PGD)
- Alternative to traditional PND for families seeking to avoid pregnancy termination.
- Method: In-vitro fertilization (IVF) followed by biopsy of a single blastomere (day 3) or trophectoderm (day 5).
- Selection: Only unaffected embryos are implanted.
Non-Invasive Prenatal Testing (NIPT)
- Emerging technology using Cell-Free Fetal DNA (cffDNA) in maternal plasma.
- Currently utilized primarily for fetal sex determination in X-linked PIDs to decide if invasive testing is necessary.
Ethical and Legal Considerations
- Genetic Counseling: Essential pre-procedure to discuss risks (0.5–1% miscarriage risk for CVS/Amniocentesis).
- MTP Act: Testing must comply with local legal frameworks regarding the termination of pregnancy for “substantial risk of serious physical or mental abnormalities.”
- Informed Consent: Must include the possibility of maternal cell contamination or inconclusive results.
Management Post-Diagnosis
- If Affected: Counseling regarding prognosis, option of termination, or preparation for in-utero therapy/early postnatal HSCT.
- If Unaffected: Routine obstetric care; confirmation of status via cord blood at birth.